Automatic focus control for cathode-ray tubes



`lune 7, 1949. A H, MANKIN 2,472,165

AUTOMATIC YFOCUS CONTROL FOR CATHODE-RAY TBES Filed April 29, 1947 Patented June 7, 1949 ATENT oFFjlfee;

Applicaitoh-zAprili29, 1947i Serial N0. 7443.586J

The, .invention .herein describedgayndiclaimed res latesetoncathodefray '.tubesnand sinrpartlcular; to meanscfor achieving-.endg maintaining-,g-,optimum focueaof.; the; electro-n1.` beamx:

The.; invention is.;.employab1e firr'cathodeeray systems# particularly. radar; fand;- television; zsy,s, tems; to A4obtain:)glatt-.ernsf.or; imagesgoff. impmved resolutieniand :excellent detalle. In nadarsytems.; iti. is important:that'ithindication one then screen be sharp and in good detail... Andeimtelevsin systems, iineigrain .pictures iofagoodresolutionpare highlydesirablw.,

In'. .accordance with theeinventiomrmeans.are providedxforidevelopingicontrolgor,correctingI .er nais iindicative f. of :the .departure-of) the'. cathode. ray.: bearnzffro'm. sharp; focus ;.,and 'inraccrdanc with-a preferred embodiment: tneseicontrolisige nals.areemploved:tofy sozadjust automaticallwthe focusing-.#rcurrents; .orf potentialsithatr optimum focusfis-fattained;

It isran :object` ofnthis .firn/entiom.y tofpxoyde meansv responsive :topvariations :inethe crosse-seni* tional arealofitheaelectrombeamifat thelscreen ofla cat-ho'de-ray:tube:..

It is another object of this-.iinventionet-tordee rivevoltagesf.fronnmeans; adapted torsbecrespnsiveztotfocal variations initiie-ebeamofraaoathodee ray tube andto utilize-saidizvcltagesfdegenerativelv tofcorrectzunwantedsdepartures ofitheabeam i rom `sharpzfocuss It isI anoth'erf Aobj ect .1 of: vthis :inventipnsto. pmi vide" automatimmeans f or obtainingsi optimum. foe cus. off-the'- electronrbeam' of. a: cathoderayitubei Another object. of this-F, inventi'omiflisfoi prof-1 vide'lmeans capable ofoperatinginla.fconventional television :receiver toy adjustl automaticallyfthe beam focusforr sharpv image-resolution andagood detail:

These and'. othery objects; advantages andifeaitures of the invention willb'e'comee clear from the i following1 ldetail'ed'- description (anddrmzthe ac'con'ipanying-l drawingsl :wherein: -1

Fiureelisla representation;- partlylschematie, partly?V diagrammatic, 'off' a rcircuit embodying# a preferred-form' ofthe-invention; ancla* F-i'gure-` -2= is fa' graphical =representaton-f3which will ilbe' helpful-fin understanding ;the invention.

Referring- -`noWffto Figure 1', tterefis' :showrrllin lthe;lower"center-1 of'tlie 'gureg'- an oscillatoiefil' whoseroutput voltagelis` Yiapplie'd alor-osea; .parallel combination; one fbranch fof Whichzcomprisesiprimaryfvvindingf-P of.; transformer Il; anditherother ybrariehfof' which.: Lcomprisescseniesecombinedrire 'sistorsi-:I 22 and .il 3i Resistorsrni': andsL dei formica voltage divider/1 by-:means oflwhichsaenortionrof the output-voltage of oscillator |07 lis,irriprges'sed upon tllecathode of tube, I4.'

Resistor i2, which will ordinarily;beconlsiclxfe;` ably. smaller in magnitude'than resistor" I3ra "i functions .as the biasing resistor for-thegtubeQbi's, being. provided by meansof cathode'ressti l2.' and bleeder resistor` 48 connectedas a..v0,lta"`ef. divider across a source ofvoltage, Th'v ues :of resistors lZand-MLare so chosemthat bias .is `as desired and relatively inc lepend'ent',l of plate-ciurent variations- The anode of :tube Mis connectedtmafsollie of positive potential, B+, by..way. .oigcoriV 1ct I5 andthe auxiliary focusing coiljif,I otcathdell, ray tube il. The function of the. .anxilielryjoo ing coil will become clear as the description 'p ,e ceeds. The main focusingy .coil |8"is connected from the source of positive potential/B, to ground by Way of variable resistor IQemplQYed-,as a manual focus control.

Cathode-ray-tube il is -conventionalin mostree spects, and in addition to focusing` coilslfand I8, vthe tube includes the vusual.filamenti l, .catlo-f ode. 22, control grid 23, fluorescent-screen, 2A; and, though not shown, conventionahhcrizontal and vertical deflection-ineens, The-dotted-.lire 25;.depicts the electron beam. f And.tlfluugli'catlfi` ode-.ray tube il is shown in llgurerl-tober-Iagmagf netically focused tube, it is to beunderstoodlthat t-hepresent invention is-applicable equally, tOQcath; ode-ray tubes having electrostatic.. focusing means.

In accordance with thepresentinvention, there is fastened to the face of tube I'la.pickup,-*eleL-l ment 2li. Pickup element 2llrmay talieyfa.-ntl-Inf.t berfof different forms. `It may comprisey-a) con@ ductive metallic plate, or' it mayfz-beamesh-` like metallic screen. It maylcovera smallpox. tion yof the face of the tube, orf-it -znavg'coyer thezentire face. In the latter'eventpelementez, Will ordinarily'have tobetranspa-renteand .may comprise, for example, a very-thinfcoatingpffgo, silver; or aluminum applied, as byfevapoiat methods, to the inner or outersurface offthegfaee of the tube.

Pickup element ZDfis electrically connected to the input circuitfoftunedlamplere p L putfcircuit of amplifier 25 `fincliidesa loadfres ter 21 :f'connected between the Virnid-point oiiisecondary Winding S of transformer Il andzglHTML--n @ppe-v sitezerrds of secondary winding AS -areicoimected asshown to the cathode of-diodef128fandft the anode of diode 29. Thegplatecircuitm 8" includes an R. C. networkcomprisin para l? combined` capacitor 3 l and resistenti-,ff ljheiath'gi 3 ode circuit of diode 29 includes R. C. network 3d comprising parallel-combined capacitor 35 and resistor 36.

The circuit elements comprising transformer l l and diodes 2li- 29, together with the associated load impedances, form a known type of phasecomparator circuit.

The output circuits of diodes 28 and 29 are connected to a voltage-combining network 38 comprised of resistors 3S and 40. These resistors are of large and equal magnitudes. The common junction of resistors 39-4 is connected to the grid of tube I4.

The arrangement thus far described comprises a preferred form of the basic automatic-focuscontrol circuit. An understanding of the practical advantages of the new circuit may, however, be best obtained by describing the operation of the circuit when employed in a television receiver, and toward that end there is included in Figure 1 several components of a conventional television receiver, together With one or two components not ordinarily included in such a receiver but desirable for best operation of the focus control system of the present invention. The circuits of all of these components are known in the television and/or radar arts and the components are therefore merely shown in block form in Figure 1 identified by appropriate descriptive markings and by reference numerals 4 l-41 inclusive.

Referring now to Figure 2, there is shown a graphical representation of the manner in which the potential, of that portion of screen 2li which is under electron bombardment, Varies in accordance with changes in the strength of the total focusing current through focusing coils l5 and E6, assuming that other factors, such as beam intensity, beam deflection speed, and incident angle of bombardment are xed. The variations in screen potential occurring in the immediate vicinity of element are picked up by the capacitive, or other, coupling action of element 2i] and these potential variations are therefore referred to in Figure 2, and hereinafter in the specification, as pickup voltages.

The graph of Figure 2 shows that, as the total focusing current is increased from a low to a high value, the pickup voltage will decrease from a high value to a minimum value and then increase again to a high Value. In Figure 2, the curve is substantially symmetrical on both sides of the point of minimum pickup voltage but it is to be understood that the curve may not be, and need not be, symmetrical in all cases. It has been observed that when the cross-sectional area of the spot is minimum, i. e., when the beam is focused most sharply upon the fluorescent screen, the pickup voltage is minimum, assuming the other factors mentioned above to have remained xe-d. And when the beam is less sharply focused, the pickup voltages are higher. The observed manner in which the potential of the bombarded area of screen, i. e. the pickup voltage, varies with variations in beam focus may be explained as follows: At sharpest focus, i. e. when the area of the beam spot is a minimum, the ratio of the number of secondary electrons knocked out of the screen by each bombarding electron is a minimum. Although minimum, the ratio is ordinarily greater than one, and since more electrons have been lost than gained, the potential of the bombarded area is positive with respect to its potential prior to bombardment. As the beam spot increases in size, the ratio of secondary electrons emitted to primary electrons arriving increases, and the bombarded area of screen becomes still more positive.

It will be understood that the extent of the departure of the beam from sharpest focus is a function of the extent of the deviation of the focusing current from optimum value. In Figure 2, the optimum focusing current is indicated as being five units, the value of the unit being purely arbitrary. If the focusing current be less than optimum, the focal point of the beam becomes located in front of the screen, i. e. on the viewing side of the screen, while if the focusing current be greater than optimum, the focal point becomes located in back of, i. e. on the gun side of the screen.

It will be observed in Figure 2 that, for values of focusing current which differ -by an equal amount from the optimum value, substantially equal voltages are derived from pickup element 20. For example, the voltage derived when the focusing current is equal to one unit is substantially equal to that derived when the focusing current is nine units. Consequently, While the voltages derived from pickup element 2u may be indicative, under the proper conditions, of the extent to which the focusing current has deviated from optimum value, the derived voltages, per se, are not indicative of the direction in which the deviation has occurred.

The present invention provides means responsive to the direction, as Well as to the magnitude, of the focal deviation. The invention contemplates modulating the focusing current in such manner as to Vary the beam focus, preferably at a relatively rapid rate, the extent of the focal excursions being so small as not to be discernible to the eye of the observer, but capable of measurement, nevertheless, by electronic means. The phase of the alternating component of the voltage picked up by element '20 is then compared with the phase of the modulating signal, and in this manner a determination may be made as to whether the beam is under-focused, over-focused, or correctly focused.

The above general remarks regarding phase comparison will become clear by considering Figure 2 wherein sine wave I1 represents the A.C. component of a current sent through auxiliary focusing coil i6 (Figure 1) at a time when the total focusing current is one unit (Fig. 2), i. e. when the D.-C. current through main focusing coil I8 plus the D.C. component of the current through auxiliary coil I6, is one unit. The beam is then under-focused. As the alternating current Ii flows through coil I 6, and the total focusing current swings back and forth about the mean value of one unit, a pickup voltage V1 is derived which is substantially 180 out of phase with the current I1.

In Figure 2, sine wave I2 represents an alterhating current, similar to I1, sent through auxiliary focusing coil I6 at a time when the beam is over-focused, as for example, when the total mean focusing current is seven units. In this case, a pickup voltage V2 is derived from element 2D which is in phase with sine wave current I2. Observe that the phase of the pickup voltage changes when the focus of the beam moves from one side of optimum focus to the other.

The operation of the circuit of Figure 1 will now be described in detail. To facilitate the rst portion of the detailed description it will be assumed that electron beam 25 is of fixed intensity and is being rapidly deected back and forth acrossxthescreen'inthe' vicinity of pickupelement 201* The'eiectupon the operationV of the focuscontrol circuit of intensity modulating the beam will'be .discussed later.

For' purposes of describing clearly the operation off'the circuit, assume the condition that a total average'focusing current of one unit (Figure 2) flws through main and auxiliary focusing coils I8 and I6. Beam 25 is therefore out of focus. Observe that a small portion of voltage W from sine wave oscillator l is impressed across cathode lad'resistor I2 of triode I4 and appears in ampli'ecl form in the plate circuit of the tube. The gain ofitube I 4 need not be high and the A.-C. component of sine wave current I1 which flows through' the auxiliary focusing coil I6 is desirably small, being of such magnitude that, although beam 25 is thereby modulated in focus, the 'focal excursions are too small for the eye to perceive. The excursions areA large enough, however, to effect variations in screen potential of such magnitude that pickup element 20 is responsive thereto.

The Variations in screen potential are, by means of element 20, capacitively coupled to the input circuitof tuned amplier 26, tuned to the frequency of the sine wave oscillator Ill. In a television,system, the frequency of sine wave oscil- Iato'r l'may desirably be of the order of ten to twenty times line frequency. In the usual television system, line frequency is approximately l kc.,.and the frequency of sine wave oscillator I0 maybe of the order of 150 to 300 kc. Either substantially lower, or substantially higher, frequencies may, however, be employed. In some cases, it may be advantageous to have the focusmodulation frequency higher than the frequency of. the highest intensity-modulation component. However, if the desiredl focus-modulation frequency-be very high, it may be necessary to tune focusing coil I6.

In Figure 2, voltage V1 represents the signal capacitivelyV picked up by element 20. As previously. indicated, the voltage V1 is substantially 180 out of phase with the sine wave variations inifocusing current, as depicted by current wave I-i in Figure 2.

The output voltage Va of tuned amplifier 26 is applied across load resistor 21 of the phasecomparator, and instantaneous voltages Va of equalmagnitude and like polarity appear at the opposing-.ends of secondary winding S of transformer II. These voltages, however, represent but one component of the total voltage appearing atteach end of secondary winding S. The other ,component comprises a voltage Ws induced in secondary Sffrom pri-mary winding P. As described earlier, the primary Winding- P of trans- 4rr'i'agnitude and opposite polarity.

It Will be seen then that the instantaneous magnitudes of the two voltages Va and Ws are `additive at one end of secondary S and subtractve at the opposite end. The constants of the syst'emmaybe so selected that the peak-to-peak lvoltage'Ws induced in secondary S from primary 'P's of` the same general order of magnitude as themaximum peak-to-peak' voltage Vil-derived from amplifier 23. When this condition obtains, the component voltages at the subtractiveend of secondary S tend to cancel completely each other and the resultant volta-ge is hence substantially zero, while at the additive end of seconda-ry S the resultant voltage isy substantially twice that of either component.

If, in the circuit of Figure 1, the outputvoltage Va of amplifier 26 is out of phase with the voltage W applied across primary windingI P, then the voltages will be additive at the lower end of secondary S, and subtractive at the upper end Assuming this to be the situation, and assumingjor purposes of illustration, the peak-to-peak amplitude of voltage Ws to be slightly greaterv than that of voltage Va, a relatively large positive'Di-C. voltage, corresponding to the sum of the'peak amplitudes of voltages Va and Ws, will beA developed across R. C. network 34 by the `peak-- detector action of diode 29 operating in cooperation with network 34; and a relatively small negative D.C. voltage, corresponding to the diierence between the peak amplitudes of voltages Va and Ws, will be developed across R. C. network 3 by the peak-detector action of'diode' 28 operating in cooperation with network 30.

The resistor and'capacitor elements comprising R. C. networks 30 and 34 are sufficiently. large to provide the desired long time constant. When the focus-control circuit is employed in a television receiver, the time constant of networks3ll and 34 may preferablyA be of the order of the duration of at least several frames.

The positive D.C. voltages developed across network 34, and the negative D.C. voltages developed across network 30; are combined and the resultante are utilized as focusing-control, i. e. focusing-correcting, voltages. In Figure 1, the said voltages are differentially combined .in voltage-combining network 38 and the resultant voltage is applied to the control grid of tube I4. In the specific situation assumed above, the negative D.C. voltage developed across network 30 is substantially smaller in magnitude than the positive D.C. voltage developed across network 34, and the arithmetical mean of the combined 'voltages is a positive voltage which is applied to the control grid of tube I4. Resistors 39.and 40 of combining network 38 are substantially larger in value than resistors 32 and 36 of R. C'. networks 30 and 34, in order not to disturb the operation of the phase comparator.

As previously indicated, tube I4 is operated-as a low-gain class A amplifier, and the effect of the application of the positive D.-C. correcting voltage to the control grid of tube I4 ismerely to increase the D.-C. component of the current flowing through auxiliary focusing coil I6; the A.-C. component is not affected to any substantial degree. The result of such an increase in total focusing current is to move the mean focus kof beam 25 in the direction of sharp or optimum focus.

And it will be evident to those skilled in the art that, by suitable selection of circuit parameters, the correcting or control voltage obtained from voltage-combining network 38 maybe of such magnitude and polarity as to move the focus of beam 25 substantially all the way to optimum focus.

Consider now the situation where the beam is over-focused. In Figure 2, voltage V2 represents the* signal derived byl element 2U when the total average focusing current is seven units and the beam is over-focused. And, as is expected from the shape of the pickup-voltage versus focusing-current curve, voltage V2 is of opposite phase to, and of smaller amplitude than, voltage V1.

When voltage V2 in amplified form is applied across resistor 21, the phase relationship between the two voltage components Va and Ws in secondary winding S is reversed, i. e. the voltage components at the upper end of secondary S are in phase and hence additive, while the voltage components at the lower end of secondary S are substantially lO" out of phase and hence subtractive. In tl'is situation, a relatively large negative D.C. voltage is developed across R. C. network 30, corrresponding to the sum of the peak amplitudes of voltages Va and Ws, by the peak-detector action of diode 28 and network 3] and a small positive D.-C. voltage, correspond- .l

ing to the difference between the peak amplitudes, tends to be developed across network 34. The negative and positive D.-C. voltages are combined in voltage-combining network 38, and the mean voltage, negative in polarity, being impressed upon the control grid of tube I4, has the eEect of decreasing the D.-C component of current flowing through auxiliary focusing coil I6. The focus of beam 25 is thereby moved from an over-focused condition to optimum condition.

Consider now the situation where beam 25 is sharply focused. In the illustration shown in Figure 2, a condition of sharp focus occurs when the total average focusing current is ve units, and the voltage then picked up by element 2! is depicted by voltage V3. Observe that the frequency of voltage V3 is twice that of the A.C. focusin -current component I3 derived from the voltage W delivered by sine wave oscillator l and applied to auxiliary focusing coil I6 by way of tube It. Since amplifier 26 is tuned to the frequency of oscillator it), the voltage V3, picked up by element 2G when beam 25 is at optimum focus, does not pass through amplifier 22 and does not appear in the output circuit thereof. Hence when beam E is at sharp focus, the voltage across secondary S comprises only that voltage induced therein from voltage lV. The instantaneous voltages at opposite ends of secondary S are then equal in magnitude and opposite in polarity.

Equal and opposite D.-C. voltages are then developed across networks 35i and 34 which, when combined in voltage-combining network 38, substantiaily cancel each other. In short, when beam 25 is at optimum focus, the correcting voltage applied to the control grid of tube I4 is of zero magnitude.

Thus far it has been assumed that electron beam 25 is not intensity modulated. However, if the focus control system is to be utilized in television systems, as is contemplated, the electron beam will ordinarily be intensity modulated during a major portion of the scanning operation; and in radar applications, also, the beam intensity varies with the strength of the reected signals. It is accordingly important that the focus control system of the present invention operate satisfactorily whether or not the beam is intensity modulated. The effect of intensity modulating the beam upon the operation of the focus control system will now be considered.

Attention is first invited to the fact that amplifier 26 is tuned to the frequency of oscillator l0. Consequently, if the frequency of oscillator ,Illl be higher than that of the highest intensitymodulating signal, for example, higher than the highest video frequency, then amplifier 26 will electively suppress all of the intensity-modulating frequencies. If the frequency of oscillator lll be lower than the highest intensity-modulating frequency, substantially all intensity-modulating frequencies, except those in the vicinity of the oscillating frequency, will be effectively suppressed, and consequently, the disturbing eiTect upon the focus control system will, in most cases, be negligible.

In those instances where the effect of intensity modulating the beam is expected to be objectionable, it will ordinarily be possible to demodulate the beam for a short period, during which time the focus control system may operate to develop the necessary control voltages.

Illustrated in Figure 1 are means readily employable in conventional television systems for providing a beam of substantially fixed. intensity during the rst few horizontal deiiections at the top of the frame, where the phosphor may either be omitted or masked.

In Figure l, block 4l represents the R.F. amplifier, video I.-F. ainplier and video detector circuits of a conventional television receiver. Block t2 represents the video amplifier, block 43 the synchronizing-separator circuit, and block 44 the ertical-synchronizing amplifier circuit, all well known in the television arts.

In a conventional television receiver, the screen is ordinarily bianked out between elds for a period equal to about sixteen horizontal lines. The sixteen lines of the vertical-blanking interval are subdivided as follows: the first three lines are occupied by the equalizing pulses; and the next three lines are occupied by the verticalsynchronizing pulse, following which the screen remains blanked for a period equal to approximately ten lines. The beam makes its vertical return sweep, i. e., the beam moves from the bottom of the frame to the top, in the three horizontal-line periods immediately following the termination of the vertical-synchronizing pulse. It will be seen then that the screen remains blanked for about seven horizontal lines following the arrival of the beam at the top of the screen. Hence, if desired, the screen may be unblanked during at least a portion of the time occupied by these seven lines, i. e., the screen may be unblanked during the period that the beam is at the top of the screen prior to the arrival of the rst video-intelligence signal for the particular field.

Referring now to Figure l, a portion of the energy of the vertical-synchronizing pulse is applied to time-delay circuit wherein a delay iS introduced equal to about three horizontal lines. The pulse delivered by time-delay circuit 4'5 is employed to trigger multivibrator circuit 4B which is arranged to deliver' a positive puise having a duration equivalent to about siX horizontal lines. The positive pulse from multivibrator 45 is combined with the output of video amplifier 42 in voltage-combining circuit vlil and the output thereof is applied to grid 23 of cathode-ray tube Il. Time-delay circuits, multivibrator circuits, and voltage-combining circuits, of the types required are all well known in the television and/ or radar arts and it is not believed necessary, therefore, either to illustrate or to describe these circuits.

By the means briey described above, screen 24 of cathode ray tube l'l is unblanked, and beam 25 is of substantially xed intensity, for a period ing circuit, for developing control voltages indicative of the magnitude and direction of the departure of the mean focus of said beam from sharp focus.

11. The combination claimed in claim 10 characterized in that means are provided for applying said control voltages to said focus-control means to alter the mean focus of said beam in such manner that said mean focus tends to correspond to sharp focus.

12. In a cathode-ray system; a cathode-ray tube having a screen, means for generating an electron beam, and focus-control means for said beam; a source of voltage having D.C. and A.-C. components; means for applying said voltage to said focus-control means in such manner that said D.C. component controls the mean focus of said beam and said A.C. component controls the frequency and extent of the excursions of said beam from said mean focus; and means, coupled to said screen, responsive to changes in screen potential resulting substantially from said excursions of said beam from mean focus for developing voltages Whose phase and magnitude indicate respectively the direction and extent of the departure of the mean focus of Said beam from sharp focus.

13. In a cathode-ray system; a cathode-ray tube having a screen, means for generating an electron beam, and means for controlling the focus of said beam; a source of voltage having D.C. and A.C. components; means for applying said voltage to said focus-control means in such manner that said D.C. component controls the mean focus of said beam and said A.C. component controls the frequency and extent of the excursions of said beam from mean focus; means, coupled to said screen, responsive to changes in screen potential resulting substantially from said excursions of said beam from mean focus for developing control voltages Whose polarity and magnitude indicate respectively the direction and extent of the departure of the mean focus of said beam from sharp focus; and means for applying said control voltages to said focus-control means to oppose said departure.

ARTHUR H. MANKIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,929,067 Hund Oct. 3, 1933 2,134,851 Bluemlein Nov. 1, 1938 2,196,838 Rogowski et a1 Apr. 9, 1940 2,219,902 Myers et a1 Oct. 29, 1940 2,310,671 Batchelor Feb. 9, 1943 2,358,901 Ziebolz Sept. 26, 1944 2,365,476 Knoop, Jr.,. et a1. Dec. 19, 1944 2,430,331 Galella et al Nov. 4, 1947 

